The present invention relates generally to micromechanical inertial sensors and, more particularly, to micromechanical inertial sensors, such as micromechanical tuning fork gyroscopes, having increased pickoff resonance damping.
Micromechanical inertial sensors are increasingly being utilized to detect rotational motions in aircraft, automobiles and other applications because of their ruggedness and small size. For example, in the automotive industry, micromechanical inertial sensors are utilized for leveling controls, anti-spin systems, anti-skid systems and navigation purposes. In the military industry, micromechanical inertial sensors are utilized for missile guidance, smart munitions, tracker buoys, autonomous vehicles, land navigation as well as a myriad of other applications. Moreover, with respect to consumer goods, micromechanical inertial sensors are utilized in virtual reality systems, camcorders, GPS receivers, and the like. In addition to their ruggedness and small size, micromechanical inertial sensors also offer potentially significant cost advantages over competing technologies.
One advantageous micromechanical inertial sensor is a tuning fork gyroscope adapted to sense rotation about a first or input axis. A tuning fork gyroscope generally includes a pair of proof masses driven to vibrate along a second axis substantially perpendicular to the first axis at a predetermined motor frequency. As a result of the manner in which the proof masses are situated with respect to an underlying substrate, movement along the second axis is typically referred to as in-plane motion. In response to rotation about the first axis, the proof masses are subjected to a Coriolis force which, in turn, causes the proof masses to move along a third axis which is substantially perpendicular to both the first and second axes, thereby producing out-of-plane motion. The tuning fork gyroscope also includes pickoff plates or sense electrodes aligned with and spaced from each proof mass such that a gap is defined between each proof mass and the respective pickoff plate. Each pickoff plate is electrically connected to a source of charge, such as a voltage source. As each proof mass moves along the third axis, such as in response to Coriolis forces, the gap between the proof mass and the respective pickoff plate will vary, thereby altering the capacitance between the proof mass and the respective pickoff plate and causing charge to move in the electrical circuit that includes the pickoff plate, i.e., in the sense circuitry. By measuring the movement of charge in the sense circuitry, the tuning fork gyroscope can provide a measurement of the capacitance between each proof mass and the respective pickoff plate and, in turn, angular rate of rotation of the tuning fork gyroscope about the first or input axis. Further details regarding tuning fork gyroscopes are provided by U.S. Pat. No. 5,920,012 to John C. Pinson, the contents of which are incorporated herein in their entirety.
In order to permit the out-of-plane motion, the proof masses are generally suspended above a substrate such that even though the proof masses are mechanically connected to the substrate, each proof mass is capable of vibration or other movement relative to the substrate. As a result of the attachment of the proof masses to the substrate, the proof masses will have both a motor natural resonant frequency for movement along the second axis and a pickoff natural resonant frequency for motion along the third axis. A tuning fork gyroscope also has a motor resonance quality factor (Q) that relates the energy required to drive the vibration of the proof mass along the second axis to the resulting movement of the proof mass at the motor frequency. In order to minimize the energy required to drive the proof mass, the motor Q is preferably as large as possible.
In response to a constant rate of rotation about the first axis, each proof mass will vibrate along the third axis at the motor frequency. In order to mechanically amplify the vibration of the proof masses along the third axis at the motion frequency, the pickoff natural resonant frequency is preferably near, but not equal to, the predetermined motor natural resonant frequency. Typically, for example, the pickoff natural resonant frequency is separated from the predetermined motor natural resonant frequency by one or two kilohertz.
If the tuning fork gyroscope rotates about the first axis at a rate of rotation that varies at a predetermined frequency, Coriolis force on the proof masses is generated simultaneously at two frequencies; namely, motor frequency plus the predetermined frequency, and motor frequency minus the predetermined frequency. Proof mass motion results at these two frequencies simultaneously. When this motion is detected and demodulated using a signal having the motor frequency for the demodulation reference, a signal at the predetermined frequency is recovered. The transfer function between the rate input, i.e., the rate of rotation about the first axis, and the demodulated output of this arrangement, as a function of the particular frequency, is one half the sum of: (1) the pickoff mode response as a function of frequency once the response has been zero frequency shifted to the modulation frequency, i.e., the motor frequency, plus (2) the pickoff mode response as a function of frequency once the response has been zero frequency shifted to minus the modulation frequency, i.e., a negative value equal in magnitude to the motor frequency. Thus there is a large response, depending on the pickoff mode Q as described below, when the particular frequency equals the difference between the motor mode and pickoff mode natural resonant frequencies. Unfortunately, the pickoff resonance has an undesirably large Q, such as 100,000. This large pickoff Q means that the tuning fork gyroscope must be designed such that the input rate transfer function peaks outside the intended input rate bandwidth of the gyroscope. Also, stringent measures must be taken to protect the gyroscope from rate inputs at frequencies equal to the difference between motor mode and pickoff mode natural resonant frequencies, so as to protect the processing electronics from saturation resulting from the very large signal levels.
In order to reduce the deleterious impact of the pickoff resonance on the movement along the third axis at the pickoff natural resonant frequency, it would therefore be desirable to reduce the pickoff Q associated with the movement of the proof mass at the pickoff natural resonant frequency. Unfortunately, prior attempts to reduce the pickoff Q have also disadvantageously reduced the motor Q. For example, the gas pressure of the atmosphere within the housing that contains the micromechanical inertial sensors has been increased in order to reduce the pickoff Q. While the increased gas pressure does reduce the pickoff Q, the motor Q is also reduced, thereby disadvantageously requiring additional energy to cause the proof mass to vibrate to the same degree.
A micromechanical inertial sensor, such as a micromechanical tuning fork gyroscope, is therefore provided that has a reduced pickoff Q, while still maintaining a relatively high motor Q. As such, the micromechanical inertial sensor of the present invention will have a increased damping at the pickoff natural resonant frequency without substantially altering the response at a predetermined motor frequency. Thus, the micromechanical inertial sensor of the present invention is less susceptible to saturation from input rates of rotation at frequencies outside the useful bandwidth of the sensor.
The micromechanical inertial sensor is adapted to sense rotation about a first or input axis. The micromechanical inertial sensor includes first and second proof masses operably connected to mechanical ground. For example, the proof masses are typically spaced apart from and mechanically connected to a substrate. The proof masses are adapted to vibrate along a second axis substantially orthogonal to the first axis. The proof masses are also moveable along a third axis substantially orthogonal to the first and second axes in response to rotation about the first axis. A micromechanical inertial sensor also includes two pickoff plates in at least partial alignment with respective proof masses. Each pickoff plate is spaced from the respective proof mass by a gap that varies in response to movement of the proof mass along the third axis. The micromechanical inertial sensor also includes voltage sources electrically connected to respective pickoff plates for providing charge to the pickoff plates. By measuring the movement of charge in each electrical circuit that includes a respective pickoff plate as the gap between the pickoff plate and the proof mass varies, a measurement of the capacitive coupling between the pickoff plate and the proof mass is obtained from which a measure of the movement of the proof mass along the third axis and, in turn, a measure of the rotation of the micromechanical inertial sensor about the first axis may be obtained.
According to the present invention, the micromechanical inertial sensor further includes resistive elements disposed in series between the voltage sources and each of the pickoff plates to increase the damping of the pickoff resonance of the micromechanical inertial sensor. In one embodiment, for example, the resistive element is selected such that the pickoff resonance has a Q of no more than 1,000. Advantageously, the resistive element causes no increase in damping of the resonance of the proof masses at the predetermined motor frequency, i.e., at the predetermined frequency at which the proof mass vibrates along the second axis. This is because the capacitance between each proof mass and the respective pickoff plate does not vary as the proof mass moves along the second axis, thus no current is caused to flow in the resistors with this motion. As such, the resistive element may be sized to between 10 kiloohms and 10 megohm and, in one advantageous embodiment, is one megohm.
The micromechanical inertial sensor can also include a plurality of drive electrodes. Correspondingly, each proof mass of this embodiment can carry a plurality of driven electrodes that are interdigitated with the drive electrodes such that electrical stimuli applied to the drive electrodes induces the vibration of each proof mass along the second axis.
By incorporating a resistive element, the pickoff resonance damping of the micromechanical inertial sensor can be increased without significantly adversely impacting the damping of the resonance at the predetermined motor natural resonant frequency. In other words, the Q of the pickoff resonance is substantially decreased, such as to 1,000 or less, without significantly decreasing the motor Q of the micromechanical inertial sensor. Thus, the pickoff resonance will continue to amplify vibration of the proof masses along the third axis at the motor frequency, such as in response to a constant rate of rotation about the first axis. However, the pickoff Q will not cause excessively large vibrations of the proof masses along the third axis in response to the rates of rotation that vary at a frequencies near the separation between the motor natural resonant frequency and the pickoff natural resonant frequency.